Wifi app for iPhone?

I understand the desire to save cost. This is just my usage experience. Instead of switching my iPhone 5 to a hotspot and using my unlimited data plan I went with a 20G Millenicon MiFi which is a Verizon product. We use about 15G per month for this product and sometimes have approached their upper limit. We do some streaming video but no movies. We have 3 cell phones and have peaked at just under 3G on them but typically stay between 1-2G.

So my message is be sure to understand what you will be doing. If just email and reading on web sites, then you may not use much. If you Skype or YouTube or watch quick help videos or news videos your usage may go up quicker than you might expect.